Generally, concrete is a brittle material with high compressive strength but low tensile strength. In the concrete industry, all concrete work is, typically, specified on the basis of the compressive strength. Any attempt to improve the crack strength (tensile strength) and toughness of the concrete almost always requires the introduction of reinforcing addition. For example, rebar (steel rods) is added which provide structural integrity but does not eliminate cracking. Metal mesh has also been added to reduce cracking but it cannot be used effectively to reinforce concrete of complex geometry.
Plastic fibers have also been used to improve the tensile strength and toughness (resistance to cracking). However, the addition of synthetic polymer fibers almost always causes a reduction in the compressive strength. In addition, when plastic fibers are used they tend to only improve either the tensile strength (strength before the first crack appears) or the toughness (resistance to cracking), but not both at the same time.
Examples of plastic fibers include polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), aramids (e.g., KEVLAR) and polyvinyl alcohol fibers. However, all of these fibers suffer from one or more problems, such as high cost, low alkaline resistance, low tenacity or low interfacial bonding between the concrete and the fiber.
Polypropylene and polyethylene have been the most preferred fiber to date due to their high tenacity and low cost. Unfortunately, these fibers suffer from very low interfacial bonding. To remedy this problem, coatings have been formed on the surface of the fibers by applying a liquid, such as glycerol ether or glycol ether on the fiber surface, as described by WO 9807668. Coatings have also been applied by vapor deposition, such as described by JP 60054950. Similarly, chemically modifying the surface has been done, such as described by JP 10236855 (treatment of the surface of a polyoxyalkylenephenyl ether phosphate and polyoxalkyl fatty acid ester). Unfortunately, these methods naturally lead to increased cost, complexity and potentially insufficient bonding of the coating to the fiber.
Another remedy has been the incorporation of inorganic particles in and on the fiber, such as described by JP 07002554. Unfortunately, the fiber process becomes much more difficult (e.g., fiber breakage) and increases the cost and, generally, decreases the tenacity of the fiber.
Further, it is known that larger fibers are preferable for improving the toughness of the concrete. Unfortunately, larger fibers further exacerbate the problem of bonding with the concrete matrix because of reduced surface area. In addition, none of these methods address another problem associated with plastic fibers in concrete, which is the tendency of larger fibers to clump together into balls that are difficult to break up when added to concrete resulting in reduced properties of the concrete.
U.S. Pat. No. 5,993,537 and WO 99/46214 describe uncontrolled fibrillation of large fibers in concrete. They describe the desirability of fibrillating large fibers into many smaller fibers and partially fibrillated fibers. They both describe that fibrillation desirably should be so great that the surface area of the fibers increase 50 fold or more. However, this extreme amount of fibrillation may lead to problems with workability, slumping, mixing and lessen desirable toughness enhancement of larger fibers.
Accordingly, it would be desirable to provide an improved fiber for improving the properties of concrete, for example, that solves one or more of the problems of the prior art, such as improving the toughness without increasing the cost of concrete when using inexpensive polypropylene fibers, while at the same time not create other problems, such as slumping and reduced workability.